1. Technical Field
This invention relates to a degassing unit, particularly that for high performance liquid chromatography, which makes it possible to perform a fast and precise eluent (solvent) delivery yet at a very small quantity by removing gaseous components dissolved in eluent (solvent) to be separated or analyzed with a liquid chromatograph.
2. Background Art
There is a tendency that high-speed (high-precision) liquid chromatography (hereinafter called HPLC) used to separate compounds in a given sample is made more and more highly accurate.
Usually in this type of HPLC, an eluent drawn from a reservoir by an eluent (solvent) delivery pump is delivered via a sample injection valve to a detection means including a separation column.
Shown in FIG. 15 is a schematic diagram explaining the configuration of HPLC. An eluent 2 in a first reservoir 1 (an eluent containing vessel) is drawn up via a pipe 3 by a pump 5 and is degassed through a degassing unit 4, and then is sent to a sample injection valve 6 (an auto sampler), to a column 7 and to a detector unit 8. The eluent delivered from the detector unit 8 is thrown out as a waste eluent 10 to a second reservoir 9. The arrow marks show the direction of the eluent delivery.
Data detected by the detector unit 8 are transferred to a data processing unit 11, wherein they are processed as required and is provided and stored in a visual form or a computer processable data form.
The column 7 is accommodated in an isothermal oven 7A to prevent the influence of external temperature. The pump 5 and the sample injection valve 6 are controlled by a system controller 12.
The degassing unit 4 installed before the pump 5 insures a stable eluent delivery and an accurate analysis by removing gases dissolved in the eluent 2 which is drawn up from the first reservoir 1 by the pump 5.
Other components consisting of this kind of high performance liquid chromatography and the functionality of the whole system are well known. So, explanations on these details are omitted here in this file.
In high speed and high accuracy liquid chromatography systems (known as semi-micro HPLC and micro HPLC) which require high accuracy in delivering the eluent under a high pressure yet at a very small quantity, it is common to install the degassing unit 4 on the inlet side of the eluent delivery pump 5 in order to insure the eluent delivery stability of the eluent delivery pump 5.
The purpose to install this type of degassing unit is to remove unnecessary gases dissolved in the eluent. The material used for degassing is a polytetra-fluoro-ethylene (PTFE) tube that is inactive to the eluent used yet good in eluent permeability. By flowing the eluent through this tube (hereinafter called PTFE tube or simply tube) and by reducing the pressure outside the tube, gaseous components dissolved in the eluent can be removed.
The stable micro eluent delivery is insured only because the eluent pump draws the eluent from which gaseous components are removed, and dispenses it.
Capillary electrophoresis chromatography (CEC) is a technology that is expected to be the next generation HPLC. This HPLC does not require an eluent delivery pump, but requires a degassing unit.
In the CEC type HPLC the eluent is transferred by the electro-osmosis flow that is generated by applying a high voltage to the column. When the high voltage is applied, the eluent is heated by joule heat, causing the temperature to rise. If oxygen (air) is dissolved in the eluent, bubbles are generated by this heat.
If bubbles are generated in the capillary column, no electric current is flown, thus making the separation impossible. The amount of eluent used in the CEC type HPLC is very small and is far smaller than that used in semi-micro HPLC. Consequently, a high-performance degassing unit that allows even smaller quantities is required.
FIG. 16 is a schematic cross section showing an example of a conventional degassing unit that uses a cylindrical capillary tube type degassing module. As explained in FIG. 15, the eluent in the first reservoir 1 is drawn via a pipe 31 by the eluent delivery pump 5. The degassing unit 4B is mounted on the pipe line 31 before the eluent delivery pump 5.
In the case of the degassing unit 4B a degassing module 16 is installed inside a vacuum chamber 13, whose air is evacuated by a vacuum pump 15, and the eluent delivery pump 5 is connected to an outlet pipe 41 of the vacuum chamber 13.
The degassing module 16 consists of gas permeable films made of a number of polytetra-fluoro-ethylene (PTFE) capillary tubes, both ends of which are bundled by multi-connectors 16a and 16b. When the eluent passes through the PTFE tubes, gases dissolved in the eluent are extracted to the vacuum chamber 13, thereby preventing gaseous bubbles from generating in the eluent delivery pump 5 while the pump 5 is drawing up the eluent.
If an attempt is made to change this degassing unit to that for micro quantity delivery, problems arise in both an internal volume of the degassing module and a structure of the module.
The internal volume of the degassing module has a standard capacity of 12ml. This volume is too large to apply to the semi-micro HPLC systems.
As the pump flow speed in semi-micro HPLC is 0.1 to 0.2 ml/min., it takes 60 to 120 minutes even in simple calculation for the eluent to pass through the degassing module. This is a considerably long time as the time of chromatographic analysis.
On the other hand, the structural problem of the degassing module is as follows. PTFE tubes, a typical degassing module material, are used in a bundle of 18 tubes with a length of 2.5 m. The eluent is distributed to each tube and flows through it.
As every tube is not the same in flow resistance, speeds for the eluent to pass through the tubes are different according to the tubes. Because of the differences of these flow speeds, the time required for the eluent to be replaced completely inside the degassing module is longer than such 60 to 120 minutes as previously mentioned.
In order to resolve such structural problem of the degassing module, I (inventor) propose the following degassing unit.
FIG. 17 shows a schematic diagram of a degassing module based on a thin film method. This figure is shown to explain another construction example of a conventional degassing unit. FIG. 18 is a cross section view of FIG. 17 taken along the line A--A in FIG. 17.
100 in FIG. 17 shows the degassing module that is a component of the degassing unit. This degassing module is accommodated in a vacuum chamber (closed console), but the illustration of the vacuum chamber is omitted here in this figure.
In FIGS. 17 and 18, this degassing module 100 is composed of two PTFE sheets 101a and 101b with a spacer 103 placed on their fringes as well as two stainless steel mesh sheets 102a and 102b put on the two PTFE sheets 101a and 101b, respectively. They are held by upper and lower holding frames 106 and 107, and are fastened in one unit by tightening bolts 108.
By means of this tightening, the fringes of the two PTFE sheets are closely contacted and form a hollow enclosure to create an internal space between them. The hollow enclosure is provided with an eluent feeding-in inlet 104 and an eluent feeding-out outlet 105 on one side (upper side of the figure) of the PTFE sheet (101a).
The eluent feeding-in inlet 104 is equipped on one short side of the rectangular PTFE sheet 101 a and the outlet 105 on its other short side, respectively, thereby securing an eluent flowing pathway between the two ports which contributes to the degassing of the eluent.
The inlet 104 and the outlet 105 are composed each of a connector 109 and its fastening part 110 both of which are fitted to an opening created by getting through the PTFE sheet 101a and the stainless steel mesh sheet 102a. Preferably, the connector 109 should be made of PTFE resin, but any material may be used if it is of organic solvent proof.
The PTFE sheet is supported by the mesh sheet of high precision stainless steel fibers of small diameter. So, it is possible to use even a thin and mechanically weak film of the PTFE sheet. As the degassing effect is in inverse proportion to the thickness of the film, the degassing efficiency can be much more improved (than with the degassing module explained in FIG. 16) by making the film thin and supporting it with the stainless steel mesh sheet.
The eluent is introduced from the inlet 104 and is transferred through the narrow internal space formed by the two PTFE sheets 101a and 101b toward the outlet 105. This eluent transfer is performed by the pumping operation of the pump connected via a pipe to the outlet 105.
While the eluent is transferred from the inlet 104 to the outlet 105, it spreads in the internal space formed by the two PTFE sheets 101a and 101b and the dissolved gaseous components are transmitted to the vacuum chamber through the walls of the PTFE sheets 101a and 101b by decreasing the pressure of the vacuum chamber by means of a vacuum pump which is not shown in the figure.
In the case of the degassing module used in this type of degassing unit, the PTFE sheets are held in the vacuum chamber and the PTFE sheet holders are comparatively large. This makes it difficult to make the area of the PTFE sheets large. Consequently, the upper limit of the degassable flow speed of the degassing unit is low, thus increasing restrictions for practical use.
As the connectors for the eluent introducing port (inlet) and the eluent discharging port (outlet) are directly provided in the PTFE sheets, it is not possible to make the gap between the two PTFE sheets smaller than a certain gap (for instance, 0.8 mm). Thus, the fact that the efficiency of the degassing module cannot be increased is the cause for the inability of increasing the flow speed.
In order to adapt the semi-micro HPLC systems or micro HPLC systems to the high flow speeds and high efficiency, the volume of the degassing unit must be made much smaller (for example, one fifth) than that of existing degassing units.
In addition, the efficiency of eluent replacement of the degassing module for the degassing unit must be improved. When the degassing unit is used for semi-micro HPLC, it is desirable that the flow rate capacity of the degassing module can cope with, at least, 1 ml/min. The degassing module with this improved capability can cope with the analysis conditions of the conventional HPLC systems.
In consideration of satisfying the above-mentioned conditions, it is impossible to realize the anticipated performance merely by making the internal volume of the degassing module with the conventional PTFE tubes smaller by one fifth. Also, it is difficult for the flat film type degassing module using the PTFE sheets to cover the analysis region of the conventional HPLC systems.